Conventionally, there is a known ad-hoc network using a proactive-type routing protocol that creates a path for each of nodes to autonomously conduct data communications, prior to data communications within the network.
Each of the nodes forming the ad-hoc network regularly exchanges with adjacent nodes messages including node information such as path information and link quality information. In addition, each of the nodes calculates communication quality of each path and, based on results of the calculation, creates a plurality of paths to the final destination and determines the optimum path. As described above, the ad-hoc network holds a plurality of paths for one destination, and thus even if the highest-quality path becomes disconnected, data can be delivered to the final destination by switching to another path to execute data transmission.
In the ad-hoc network, in the event of congestion, the frequency of occurrence of collisions becomes higher to increase the frequency of re-transmission. Accordingly, an increased amount of time is taken for data transmission/reception between nodes existing on the path from the transmission source node to the final destination, which causes a phenomenon that the data does not reach the final destination within an expected time. In the event of such a phenomenon, even emergency data with higher priority than normal data is not transmitted/received.
In recent years, there has been known a method for avoiding occurrence of congestion by specifying at the data transmission source a timing for data transmission at the opposed side to prevent collision of data transmitted from nodes. In addition, an upper limit is placed for each node on data transmission bandwidth capable of being occupied by a connection during data transmission, and when data is to be transmitted with a data transmission bandwidth beyond the upper limit, a priority connection capable of occupying the data transmission bandwidth is established for data transmission. In addition, there has been known a control technique for, while a priority connection is established, data transmission by other connections is temporarily interrupted, whereas emergency data with a higher priority can be transmitted by other connections.
Patent Literature 1: International Publication Pamphlet No. WO 2011/013165
Patent Literature 2: Japanese Laid-open Patent Publication No. 2004-180131
Patent Literature 3: Japanese Laid-open Patent Publication No. 2009-130859
However, even with the use of the conventional technique, there is a problem that emergency data with a higher priority is delayed or does not reach during congestion.
For example, the method for specifying a timing for data transmission is a technique for preventing occurrence of congestion, and thus in the event of congestion, not only emergency data but also normal data may be delayed or not reach the destination. In addition, according to the technique for transmitting emergency data even while a priority connection is established, the priority connection occupies the data transmission bandwidth to narrow the bandwidth for transmission of the emergency data. As a result, the emergency data may be delayed or not reach the destination, depending on the volume of the emergency data and the like.